The present invention relates generally to electronic circuits, and in particular to common mode noise in differential amplifiers.
Differential amplifiers are widely used in electrical circuits to amplify a difference in voltages between two input signals to produce amplified differential output signals.
FIG. 1 shows a typical differential amplifier 100 having driver transistors N1 and N2, load transistors P1 and P2, and a current source N3. Differential amplifier 100 receives differential signals INA and INB and outputs differential output signals OA and OB.
In most electronic circuits, a noise at the input signal can affect the output signal. In differential amplifier 100, since the INA and INB are differential signals, a noise that is common to the INA and INB signals may be absent from the differential component of the OA and OB signals. Although the common mode noise is absent from differential component of OA and OB signals, high frequency component of the common mode noise can still be transmitted to the outputs in common mode. This can create a problem when differential amplifier 100 drives a non-differential element. For example, FIG. 1 shows differential amplifier 100 driving an inverter 102.
FIG. 2 shows various signals for FIG. 1 in which differential amplifier 100 operates in two exemplary conditions; one condition has no noise and the other includes noise. As shown in FIG. 2, the transition point of the OUT signal of inverter 102 shifts by a time T because of the effect of the common node noise. This time shift may cause a circuit having differential amplifier 100 and inverter 102 to perform improperly.
Therefore, in some cases, differental amplifier 100 is unsuitable for driving a non-differential element. Conventional methods use various types of feedback to compensate the common mode noise or voltage shifts. This feedback usually requires that some of the common mode noise or the voltage shift be seen at the output before the proper compensation can be applied. This is inherently slow relative to some common mode noise that can be experienced.
The various embodiments of the present invention provide methods and circuits for reducing the common mode noise effect of differential amplifiers through a feed forward approach. This feed forward approach has the benefit of compensating nearly instantaneously. Therefore, much higher frequency common mode noise may be compensated.
In one aspect, a circuit includes a differential amplifier having a first differential input node and a second amplifier input node for receiving differential input signals, and having a first differential output node and a second differential output node for outputting differential output signals. An isolation unit connects in series with a supply path of the differential amplifier between a supply node and a compensation node. A capacitive network connects to the compensation node and the first and second differential amplifier input nodes. The isolation unit and the capacitive network form a compensation unit for reducing the effect of the common mode noise on the differential output signals. The capacitive network provides a feed forward path for the compensation.
In another aspect, a method of reducing the effect of the common mode noise includes receiving differential input signals at a plurality of input nodes of a differential amplifier. The method also includes generating differential output signals at a plurality of output nodes of the differential amplifier. The method further includes compensating a common mode noise affected on the differential output signals.